/Engine/Source/Runtime/Core/Public/Containers/Map.h
The association doesn't replace any of the key's existing associations. However, if both key and value match an existing association in the map, no new association is made and the existing association's value is returned.
A reference to the value as stored in the map; the reference is only valid until the next change to any key in the map.
template<typename InitKeyType, typename InitValueType> ValueType & EmplaceUnique ( InitKeyType && InKey, InitValueType && InValue )